The present invention relates to a nuclear medicine diagnostic apparatus, and more particularly, to a positron emission tomography (hereinafter referred to as “PET”) apparatus, which is a kind of a nuclear medicine diagnostic apparatus using a semiconductor radiation detector, semiconductor radiation detection apparatus or detector unit.
A detector using a NaI scintillator is known as a conventional radiation detector for detecting radiation such as γ-rays. With a gamma camera (a kind of nuclear medicine diagnostic apparatus) provided with a NaI scintillator, radiation (γ-rays) incident on the scintillator at an angle restricted by many collimators interacts with NaI crystals and emits scintillation light. This light travels in such a way as to sandwich a light guide, reaches a photoelectric multiplier and becomes an electrical signal. The electrical signal is shaped by a measuring circuit mounted on a measuring circuit fixing board and transferred from an output connector to an external data collection system. All these scintillator, light guide, photoelectric multiplier and measuring circuit, measuring circuit fixing board, etc., are housed in a light shielding case and shielded from electromagnetic waves other than external radiation.
Since a gamma camera using a scintillator has a structure with a large photoelectric multiplier (also called “photomultiplier”) placed after one large crystal such as NaI, its position resolution remains on the order of 10 mm. Furthermore, since the scintillator detects radiation in multi-stages of conversion from radiation to visible light, from visible light to electrons, it has a problem of having considerably poor energy resolution. For example, there is a PET apparatus (positron emission tomography apparatus) having position resolution of 5 to 6 mm or a high-end PET apparatus having position resolution of 4 mm or so, but since their photoelectric multipliers use vacuum tubes, it is difficult to further improve position resolution.
There are radiation detectors for detecting radiation according to principles different from those of such a scintillator, such as semiconductor radiation detectors provided with a semiconductor radiation detection element using a semiconductor material such as CdTe (cadmium telluride), TlBr (thallium bromide) and GaAs (gallium arsenide).
This semiconductor radiation detector is attracting attention because its semiconductor radiation detection element converts electrical charge produced by interaction between radiation and the semiconductor material to an electrical signal, and therefore it has better efficiency of conversion to an electrical signal than the scintillator and can also be miniaturized.    [Patent Document 1] JP-A-2003-79614 (paragraph No. 0016)    [Patent Document 2] JP-A-2003-167058 (paragraph No. 0020, 0023)
Meanwhile, when a semiconductor material such as T1 making up a semiconductor radiation detection element interacts with radiation in a semiconductor radiation detector, holes having positive electrical charge and electrons having negative electrical charge are generated. While mobility of electrons is relatively large, mobility of holes is relatively small. That is, electrons move relatively easily and holes move with difficulty. This takes more time for holes to reach an electrode than electrons. Moreover, holes may be annihilated before reaching the electrode. This involves a problem that the detection sensitivity of radiation is worsened. Thus, these problems require solutions.
It is an object of the present invention to provide a semiconductor radiation detector capable of improving detection sensitivity.